1. Field of the Invention
The present invention relates to an antenna capable of adjusting impedance matching, and more particularly, to an antenna utilizing a matching circuit for adjusting the impedance matching.
2. Description of the Prior Art
As wireless telecommunication develops with the trend of micro-sized mobile communication products, the location and the space arranged for antennas are limited. Therefore, some built-in micro antennas have been developed. Currently, some micro antennas such as a chip antenna, a planar antenna and so on are commonly used. All these antennas have the feature of small volume. Additionally, planar antennas are also designed in many types such as microstrip antennas, printed antennas and planar inverted F antennas. These antennas are widespread applied to GSM, DCS, UMTS, WLAN, Bluetooth, etc.
Please refer to FIG. 1, which is a diagram of a dual-frequency antenna 10 in the prior art. The dual-frequency antenna 10 includes a substrate 12, a radiation element 14, a connection element 16, and a feed element 18. The substrate 12 approximately is a rectangle, and has a first side 122 and a second side 124. The first side 122 includes a short point 126 and a grounding point 128. The radiation element 14 is installed on the first side 122. The radiation element 14 includes a first radiator 141, a second radiator 142, and a first metal arm 143. The first radiator 141 approximately parallels the first side 122. The second radiator 142 approximately parallels the first side 122 and is extended in a direction opposite to the first radiator 141. A rear end of the first radiator 141 and a rear end of the second radiator 142 each comprise a bending 146 and 148 used for individually increasing radiation efficiency of the first radiator 141 and the second radiator 142. The first metal arm 143 is approximately perpendicular to the first side 122 and has a first end 144 coupled to a joint of the first radiator 141 and the second radiator 142, and a second end 145. The feeding element 18 is coupled between the second end 145 of the first metal arm 143 and the grounding point 128. The connection element 16 is approximately an L shape and has a first end 163 coupled to the second end 145 of the first metal arm 143, and a second end 165 coupled to the short point 126 of the substrate 12.
As shown in FIG. 1, due to a length of the first radiator 141 being greater than a length of the second radiator 142, signals of a first resonance mode (low frequency) can be resonated by the first radiator 141 and signals of a second resonance mode (high frequency) can be resonated by the second radiator 142. A sum of the length of the first radiator 141 and a length of the first metal arm 143 is approximately one-fourth of a wavelength of the first resonance mode generated by the dual-frequency antenna 10(λ/4). A sum of the length of the second radiator 142 and the length of the first metal arm 143 is approximately one-fourth of a wavelength of the second resonance mode generated by the dual-frequency antenna 10. The substrate 12 comprises dielectric material or magnetic material and is coupled to a system ground terminal (GND). The radiation element 14 and the connection element 16 are each substantially composed of a single metal sheet.
Please refer to FIG. 2 and FIG. 1. FIG. 2 is a diagram illustrating the VSWR (voltage standing wave ratio) of the dual-frequency antenna 10 in FIG. 1. The horizontal axis represents frequency (GHz) that distributes from 0.7 GHz to 2.5 GHz, and the vertical axis represents VSWR defined by an equation of VSWR=Vmax/Vmin. As shown in FIG. 2, the frequencies and the VSWR of eight points are marked, for example, the frequency of the point 1 is about 0.826 GHz and its VSWR is about 3.503; the frequency of the point 8 is about 2.17 GHz and its VSWR is about 1.943. Thus it can be seen that the bandwidth of the first resonance mode generated by the dual-frequency 10 falls in the neighborhood of 900 MHz, and the bandwidth of the second resonance mode falls in the neighborhood of 1900 MHz.
Nowadays, notebook computers have become one of the common electronic consumer products in human life. The ability to enter a network through wireless local area networks (WLAN) has become a standard equipment of the notebook computers. It is impossible to enter the network wirelessly if lying in an environment without the wireless local area networks. Hence, an idea of making the notebook computers enter the network wirelessly and speedily through mobile base stations grows in abundance and somewhat suddenly. Thus antennas should not only conform to operational bandwidths of wireless local area networks but also conform to operational bandwidths of wireless wide area networks (WWAN). How to reduce sizes of the antennas, improve antenna efficiency, and improve impedance matching becomes an import topic of the field.